Washington (CNN) – The chairwoman of the Senate Intelligence Committee is painting a grim picture of the U.S. effort in Afghanistan, but Sen. Dianne Feinstein, D-California, insists that the effort must continue.

“Failure is not an option,” Feinstein said in an interview broadcast Sunday on CNN’s “State of the Union.”

Feinstein agreed that the training of the Afghan army continues to face significant challenges, but she said the Afghanistan-Pakistan theater is too strategically important to U.S interests not to continue the U.S. military operation.

“Also, there's one, I think, irreversible truth - the Taliban is on a march,” Feinstein also told CNN Chief Political Correspondent Candy Crowley. “If you lose Afghanistan, Pakistan is the next step.”

She added, “So the question becomes, either the Taliban becomes a force for good, participates in government - we're not there yet - or it has to be defeated.”

Sen. Richard Lugar of Indiana, the Ranking Republican on the Senate Foreign Relations Committee who joined Feinstein during the interview, agreed that training Afghan security forces remains a challenge.
“Well, we don't say good-bye [to the Afghans], we say right now to the Afghans that we want to train you so that you are able to police your own territory in order to govern,” Lugar told Crowley. “Now, as Dianne Feinstein has said, this is tough to do. You have almost everybody who is an illiterate to begin with. The allies that we had hoped for to send trainers haven't sent very many. Our own trainers are - are too few.

“So, as a result, this is going more slowly. I sympathize with General McChrystal and General Petraeus, as people press them for dates. They're saying ‘one thing at a time.’ We've really got to get the training done. It's going more slowly.”

Gen. Stanley McChrystal, the top U.S. military commander in Afghanistan, recently said that American-led operations against the Taliban in southern Afghanistan will happen "more slowly than we had originally anticipated."

And testifying before a congressional committee late last week, Gen. David Petraeus, who heads the U.S. Central Command, said "The conduct of a counterinsurgency operation is a roller-coaster experience. There are setbacks as well as areas of progress or successes. But the trajectory in my view has generally been upward, despite the tough losses, despite the setbacks."